## When Should I Use io.TeeReader vs io.Copy in Go?

Understanding the Distinction Between io.TeeReader and io.Copy in Go

In the Go programming language, both io.TeeReader and io.Copy are frequently used for reading data from a source and writing it to a destination. However, there are subtle differences between these two functions that can impact your code's behavior.

io.Copy: Direct Data Transfer

io.Copy performs a direct data transfer from an input reader (io.Reader) to an output writer (io.Writer). The function doesn't provide any way to access the copied data; it simply forwards it from one stream to another. This behavior is ideal when you need to move data efficiently without any modifications.

io.TeeReader: Data Teeing for Parallel Processing

In contrast to io.Copy, io.TeeReader creates a new io.Reader that wraps around the original reader. This new reader provides parallel access to the data stream. As you read from the teed reader, the data is simultaneously written to the provided io.Writer.

Benefits of io.TeeReader

This feature makes io.TeeReader particularly useful when you need to perform additional operations on the data while also writing it to an external destination. For instance, you could use io.TeeReader to calculate the MD5 hash while copying data to the console.

Example Usage

Here's an example that demonstrates the use of io.TeeReader:

<code class="go">package main

import (
    "bytes"
    "fmt"
    "hash/crc32"

    "io"
)

func main() {
    // Create a source reader
    input := "Hello, World!"
    reader := bytes.NewReader([]byte(input))

    // Create a hash and a teed reader
    hasher := crc32.NewIEEE()
    teedReader := io.TeeReader(reader, hasher)

    // Read from the teed reader while copying to the hasher
    if _, err := io.ReadAll(teedReader); err != nil {
        panic(err)
    }

    // Retrieve the hash value
    hashValue := hasher.Sum32()

    fmt.Printf("Copied and hashed \"%s\": %x\n", input, hashValue)
}</code>

Conclusion

io.TeeReader and io.Copy offer distinct strategies for data handling in Go. io.Copy efficiently transfers data directly, while io.TeeReader allows simultaneous data processing and writing. Understanding the difference between these functions will help you choose the right approach for your specific scenarios.

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